Solar Power System using Hybrid Trough and Photovoltaic Two-Stage Light Concentration

ABSTRACT

A solar power method is provided using two-stage light concentration to drive concentrating photovoltaic conversion in conjunction with thermal collection. The method concentrates light rays received in a plurality of transverse planes towards a primary linear focus in an axial plane, which is orthogonal to the transverse planes. T band wavelengths of light are transmitted to the primary linear focus. R band wavelengths of light are reflected towards a secondary linear focus in the axial plane, which is parallel to the primary linear focus. The light received at the primary linear focus is translated into thermal energy. The light received at the secondary linear focus is focused by optical elements along a plurality of tertiary linear foci, which are orthogonal to the axial plane. The focused light in each tertiary primary focus is focused into a plurality of receiving areas, and translated into electrical energy.

RELATED APPLICATIONS

This application is a Continuation of an application entitled HYBRIDTROUGH SOLAR POWER SYSTEM USING PHOTOVOLTAIC TWO-STAGE LIGHTCONCENTRATION, invented by Wheelwright et al., Ser. No. 14/503,822,filed on Oct. 1, 2014.

This invention was made with Government support under DE-AR0000465awarded by DOE. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to solar generated power and, moreparticularly to a hybrid system that combines thermal and photovoltaicfor energy generation and thermal storage.

2. Description of the Related Art

FIG. 1 is a solar energy collection device using a parabolic trough(prior art). The design of Brunotte et al. [1] is one of the earliest toattempt to convert a line focus into a series of higher concentrationfoci. In this design, a parabolic trough is used to illuminate a seriesof solid compound parabolic concentration (CPC) secondaries, eachconjugate to a single photovoltaic (PV) cell. The trough is trackedabout a polar-aligned axis, so the range of skew angles is limited to+/−23.5° throughout the year. The CPCs have square or rectangularapertures, and may have an asymmetric acceptance angle. The acceptanceangle in the transverse plane of the trough is determined by the maximumrim angle of the trough. If an asymmetric trough is used, as shown, thenthe CPCs can be tilted to a median angle, and the acceptance angle isonly half the rim angle of the trough. In the other dimension (along theaxial plane of the trough), the CPCs are required to have an acceptanceangle of +/−23.5°, to maintain seasonal performance.

FIGS. 2A and 2B depict a design of tracking secondaries for conventionaltroughs, off-axis troughs, and Cassegrain troughs (prior art). Cooper etal. [3] and Thesan S.p.a. [4] both employ a second degree of trackingfreedom. In addition to single-axis trough tracking, the secondaryoptics are allowed to rotate or translate to compensate for the changingincidence angle (within the axial plane of the trough). The secondariesinclude hollow, rotating CPCs and solid dielectric reflectors. A designwith an array of hollow CPCs operates near the prime focus.Alternatively, a laterally-translated cylinder or spherical lenses maybe used [4].

FIG. 3 is a partial cross-sectional view depicting reflected andtransmitted wavelength bands of a M2 spectrum splitter (prior art). Inprinciple, such a design permits ultraviolet (UV) and infrared (IR)light to be collected as thermal energy at heat receiver 1 (HR1), andvisible (Vis) and near infrared (NIR) to be collected as photovoltaicenergy. A Cassegrain geometry poses a new obstacle to achieve highconcentration. Since Cassegrain optics have a large focal length, thesolar image formed below M2 is likewise larger. The primaryconcentration from a Cassegrain trough is thus lower than it would be atthe prime focus. In order to keep the solar image small after theCassegrain M2, the M2 size should be very large. However, this causes M2to cast a large shadow on M1 (the trough). Alternatively, M2 can be madevery small so that there is minimum shadowing effect. However, the sizeof the solar image at the base of the trough becomes very large. Analternative approach would require raising the receiver (photovoltaiccells) to be closer to M2. This allows some reduction in the focallength. However, this may affect trough stability due to a raised centerof gravity. Therefore, without additional concentration, this design isimpractical for both concentrated photovoltaic (CPV) and concentratedsolar power (CSP) purposes.

U.S. Pat. No. 5,505,789 uses a tessellating line focus with solidsecondary funnels to address the above-mentioned problems associatedwith Cassegrain optics [5]. U.S. Pat. No. 5,505,789 discloses line-focuslenses and a line-focused PV module. The whole system is an array oflinear arched Fresnel lenses with a linear PV cell receiver locatedalong the focal line of each lens. The photovoltaic cell receiverconsists of high efficiency cells interconnected in a string with asolid secondary optical element adhesive bonded to the cells. Theentrance aperture of each secondary optical element is rectangular inshape and the optical secondaries are butted up against each other in aline to form a continuous entrance aperture along the focal line. Inaddition to providing more concentrated sunlight, the solid opticalsecondaries shield the cells from air, moisture, and contaminants, andto a lesser extent against radiation damage. However, since this systemdoes not employ Cassegrain optics or an additional means ofconcentrating light to the PV cells, it is a low concentrated CPVsystem. It is not obvious that this system can be modified to useCassegrain optics, or that the light collected in such a system can beconcentrated sufficiently for PV collection, in light of all the reasonsmentioned above.

Other beam splitting approaches for solar power include Imenes et al.[6], dichroic filter designs for hybrid solar energy by DeSandre et al.[7], analysis of hybrid solar energy efficiencies by Hamdy et al. [8],and designs of hybrid solar systems by Soule et al. [9, 10].

FIG. 4 is a partial cross-sectional view of a Cassegrain hybrid troughsystem with PV at the bottom of the trough [11] (prior art). A similarCassegrain trough system with beam splitter, but with no concentrationat PV cells in a slit at vertex of trough is described by Jian et al.[12].

It would be advantageous if a hybrid solar system using Cassegrainoptics could be designed to optimally collect both thermal and PVenergy.

[1] “Two-stage concentrator permitting concentration factors up to 300×with one-axis tracking”, Brunotte, M., Goetzberger, A., & Blieske, U.(Jan. 1, 1996). Solar Energy, 56, 3, 285-300.

[2] “BICON: high concentration PV using one-axis tracking and siliconconcentrator cells”, Mohr, A., Roth, T., & Glunz, S. W. (Jan. 1, 2006).Progress in Photovoltaics, 14, 7, 663-674.

[3] “Theory and design of line-to-point focus solar concentrators withtracking secondary optics”, T. Cooper. G. Ambrosetti, A. Pedretti, andA. Steinfeld, Appl. Opt. vol. 52, 8586-8616 (2013).

[4] “Solar Receiver for a Solar Concentrator with a Linear Focus”, A.Balbo Divinadio and M. Palazzetti, Thesan S.p.a., US 2011/0023866,Published Feb. 3, 2011.

[5] “Line-focus photovoltaic module using solid optical secondaries forimproved radiation resistance”, L. M. Fraas and M. J. Oneill, EntechInc., U.S. Pat. No. 5,505,789, Granted Apr. 9, 1996.

[6] “Spectral beam splitting technology for increased conversionefficiency in solar concentrating systems: a review”, A. G. Imenes andD. R. Mills. Solar Energy Materials & Solar Cells. Vol. 84, pp 19-69(2004).

[7] “Thin-film multilayer filter designs for hybrid solar energyconversion systems”, L. DeSandre, D. Y. Song, H. A. Macleod, M. R.Jacobson, and D. E. Osborn, Proceedings of the SPIE Vol. 562, pp 155-159(1986).

[8] “Spectral selectivity applied to hybrid concentration systems”, M.A. Hamdy, F. Luttmann, D. E. Osborn, M. R. Jacobson, and H. A. Macleod,Proceedings of the SPIE Vol. 562, pp 147-154 (1986).

[9] “Efficient hybrid photovoltaic-photothermal solar conversion systemwith cogeneration”, D. E. Soule, E. F. Rechel, D. W. Smith, and F. A.Willis, SPIE Vol. 562, pp 166-173 (1985).

[10] “Heat-Mirror Spectral Profile Optimization for TSC Hybrid SolarConversion”, D. E. Soule and S. E. Wood, SPIE Vol. 653, p 172-180(1986).

[11] “Bandwidth and angle selective holographic films for solar energyapplications”, C. G. Stojanoff, J. Schulat, and M. Eich, SPIE Vol. 3789,pp 38-49 (1999).

[12] “Optical modeling for a two-stage parabolic trough concentratingphotovoltaic/thermal system using spectral beam splitting technology”,S. Jian, P. Hu, S. Mo, and Z. Chehn, Solar Energy Materials and SolarCells vol. 94 168-1696 (2010).

SUMMARY OF THE INVENTION

Hybrid solar generators employing a dichroic spectrum splitter, and theproblems associated with these designs have been explored extensively,as described above in the Background Section. The system describedherein improves upon conventional methods using a low-concentration linefocus from the Cassegrain trough, by further concentrating to higherlevels, the focused sunlight required by concentrated photovoltaic (CPV)cells. Along a line focus, sunlight that has already been concentratedin one dimension is difficult to further concentrate in that samedirection. However, significant additional concentration is possible inthe direction orthogonal to the line focus. This leads to a two-steparchitecture, where the final concentration is a result of twoorthogonal operations.

A Cassegrain solar concentrator is used to split the solar spectrum intotwo bands. T band wavelength light, e.g., ultraviolet (UV) and infrared(IR) light, is allowed to pass through a dichroic secondary mirror (M2)and is absorbed by a standard thermal receiver. The R band wavelengthlight (e.g., visible (Vis) and near IR (NIR) light) is reflected by M2,forming a medium-concentration solar image below M2. The focal length ofthe Vis-NIR band is longer than that of the UV/NIR band. In the absenceof the system disclosed herein, the Vis-NIR focus is unsuitable for CPVapplications. The only potentially economically viable option would beto cover the entire Vis-NIR focus area with single-junction (i.e. p-njunction) PV cells. By introducing an array of rotating refractiveoptics near the medium-concentration line focus, additionalconcentration can be gained in the direction orthogonal to the originalconcentration.

Thus, the hybrid CPV system converts solar power into both electricityand thermal energy by splitting the solar spectrum into two wavelengthbands. The visible and near infrared spectra are used for directelectricity production through CPV devices, taking advantage of maximumCPV efficiency in this wavelength band. The ultraviolet and infraredspectra are used for thermal energy collection, where the heat isconveyed to a central power block via a heat transfer fluid (HTF). Thus,the hybrid system can deliver higher energy (combined electrical andthermal energy) than either a CPV or concentrated solar power (CSP)system alone. That is, the hybrid system not only generates variableelectricity but also produces dispatchable thermal energy for low costand high capacity thermal storage. The added-on storage capacity,besides electricity generation, poises the hybrid system to be awell-balanced power generation system. Dispatchability addresses gridneeds by delivering power when the demand is high. Thus, a grid tiedhybrid system can sell electricity at the peak price to maximizerevenues for utility companies and reduce fossil fuel consumption.

Accordingly, a solar power method is provided using two-stage lightconcentration to drive CPV conversion in conjunction with thermalcollection. The method concentrates light rays received in a pluralityof transverse planes towards a primary linear focus in an axial plane,which is orthogonal to the transverse planes. T band wavelengths oflight are transmitted to the primary linear focus. R band wavelengths oflight are reflected towards a secondary linear focus in the axial plane,which is parallel to the primary linear focus. The light received at theprimary linear focus is translated into thermal energy. The lightreceived at the secondary linear focus is focused by a plurality ofoptical elements aligned along the secondary axis, into a plurality oftertiary linear foci, which are orthogonal to the axial plane. The lightfocused by the optical element in each tertiary primary focus isconcentrated into a plurality of receiving areas, and translated intoelectrical energy.

In one aspect, a reflective trough, having a primary axis and aparabolic curved surface, concentrates light rays received in theplurality of transverse planes towards the primary linear focus. Adichroic spectrum splitter having a hyperbolically curved surface, anaxis aligned in parallel to the primary linear focus, and a positionbetween the secondary linear focus and the primary linear focus,transmits the T band wavelengths of light and reflects the R bandwavelengths light. A plurality of optical elements focuses the R bandwavelengths of light received at the secondary linear focus along acorresponding plurality of tertiary linear foci. A plurality of opticalfunnels concentrates the focused light in each tertiary primary focusinto a corresponding plurality of receiving areas.

Additional details of the above-described method and a hybrid troughsolar power system are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a solar energy collection device using a parabolic trough(prior art).

FIGS. 2A and 2B depict a design of tracking secondaries for conventionaltroughs, off-axis troughs, and Cassegrain troughs (prior art).

FIG. 3 is a partial cross-sectional view depicting reflected andtransmitted wavelength bands of a M2 spectrum splitter (prior art). FIG.4 is a partial cross-sectional view of a Cassegrain hybrid trough systemwith PV at the bottom of the trough [11] (prior art).

FIG. 5 is a perspective view of a hybrid trough solar power system usingtwo-stage light concentration to drive concentrating photovoltaic (CPV)conversion in conjunction with a thermal collector.

FIGS. 6A and 6B are partial cross-sectional views of the dichroicspectrum splitter.

FIGS. 7A through 7C are views of an exemplary concentrating opticssection.

FIGS. 8A through 8D depict exemplary optical element forms.

FIGS. 9A through 9F depict exemplary optical funnel designs.

FIG. 10 is a perspective drawing depicting an array of optical funnelswith varying aperture lengths. FIGS. 11A through 11D describe the systemof FIG. 5 from a different perspective.

FIGS. 12A through 12D depict the system of FIGS. 11A through 11Doperating at different angles of the sun.

FIG. 13 is a flowchart illustrating a solar power method using two-stagelight concentration to drive CPV conversion in conjunction with thermalcollection.

DETAILED DESCRIPTION

FIG. 5 is a perspective view of a hybrid trough solar power system usingtwo-stage light concentration to drive concentrating photovoltaic (CPV)conversion in conjunction with a thermal collector. The system 400comprises a reflective trough 402 having a primary axis 404 and aparabolic curved surface 406 for concentrating light rays (e.g., lightrays 408 a 1 through 408 c 2) received in a plurality of transverseplanes into a primary linear focus 410 in an axial plane (not shown),orthogonal to the transverse planes. For example, light rays 408 a 1 and408 a 2 are in one transverse plane and light rays 408 c 1 and 408 c 2are in another transverse plane. In one aspect, the reflective trough402 is rotatable about the primary axis 404.

A dichroic spectrum splitter 416 has a hyperbolically curved surface418, an axis 420 aligned in parallel to the primary linear focus 410,and a position between the reflective trough 402 and the primary linearfocus 410. In one aspect, the T band wavelengths of light include bothwavelengths greater than near infrared (NIR) and less than NIR, and theR band wavelengths include NIR wavelengths of light.

FIGS. 6A and 6B are partial cross-sectional views of the dichroicspectrum splitter 416. Light rays accepted by the reflective trough 402in transverse plane 414 are reflected to the dichroic spectrum splitter,as represented by rays 500. The dichroic spectrum splitter 416 transmitsT band wavelengths of light 422, and reflects R band wavelengths light502 to a secondary linear focus 426 formed parallel to a vertex 506 ofthe reflective trough 402 in the axial plane 504. In this example, thesecondary linear focus 426 and the vertex 506 are collocated, and theaxial plane 504 is plane facing the reader in FIG. 6B (i.e. the sheetupon which FIG. 6B is formed). A thermal collection tube 430 (HR1) isaligned along the primary linear focus 410 for the light transmitted bythe dichroic spectrum splitter 416. For simplicity, the thermalcollection tube 430 is shown aligned along the primary linear focus 410.

Returning to FIG. 5, a plurality of concentrating optics sections 432are formed in series along the secondary linear focus 426. Ideally, thesecondary linear focus would be a narrowly focused line in the axialplane. However, due to the longer focal length of the Cassegrain opticalpath, the secondary linear focus 426 is of lower concentration than theprimary linear focus 410. The secondary linear focus is represented hereas a narrow plane transverse to the axial plane. As explained below,concentrating optics sections are used to boost the concentration of thesecond linear focus 426.

FIGS. 7A through 7C are views of an exemplary concentrating opticssection 432. Each concentrating optics section 432, also referred toherein as concentrating lens (CL) optics, comprises an optical element600 for focusing the R band wavelengths of light reflected by thedichroic spectrum splitter along a tertiary linear focus 602, orthogonalto the axial plane. A plurality of optical funnels 604 concentrate the Rband wavelengths of light focused by the optical element 600 to acorresponding plurality of receiving areas 606. A plurality of PVdevices 608 each have an optical interface formed at a correspondingreceiving area 606. In one aspect, the PV devices 608 are multi-junctioncells, each junction having an energy bandgap converting R bandwavelengths of light to electrical current. For example, if the dichroicspectrum splitter reflects light in the R band of wavelengths between500 and 810 nanometers (nm), then the PV devices 608 may be doublejunction tandem cells with energy bandgaps of 1.88 electron volts (eV)and 1.43 eV, or triple junction tandem cells with an energy bandgaps of2.05 eV, 1.77 eV, and 1.43 eV. Alternatively, if the dichroic spectrumsplitter reflects light in the R band of wavelengths between 650 and 850nm, the PV devices 608 may be single junction cells with an energybandgap of 1.43 eV. In another aspect, if the dichroic spectrum splitterreflects light in the R band of wavelengths between 700 and 1000 nm,then the PV devices 608 may be single junction cells with an energybandgap of 1.1 eV.

Each optical element 600 has an optical input aperture 610 elongatedorthogonal to the axial plane. Likewise, each optical funnel 604 in theconcentrating optics section 432 has an optical input aperture 612underlying the lens 600 and elongated orthogonal to the axial plane.

In one aspect, each optical element 600 has an optical input aperturefirst axial plane-width 614 in an aperture (e.g., horizontal) plane,where the aperture plane is orthogonal to the axial plane. Each opticalelement 600 focuses light along the tertiary linear focus 602,coincident with the elongated optical funnel input apertures 612 in theaxial plane, to a second axial plane-width 616, smaller than the firstwidth 614. In another aspect, each concentrating optics section 432 isrotatable about an axis (e.g., in the tertiary plane, see FIG. 7A)formed orthogonal to the axial plane.

Returning briefly to FIG. 5, a thermal collector or thermal coolingblocks, represented by reference designator 434 (HR2), may optionallyattached to the PV devices.

FIGS. 8A through 8D depict exemplary optical element forms shapes. FIG.8A depicts a linear Fresnel lens 600, FIG. 8B depicts an acylindricallens 600, and FIG. 8C depicts a cylindrical lens 600. A Fresnel lens canbe inexpensively hot embossed in a plastic, such as poly(methylmethacrylate) (PMMA). However, Fresnel lenses have losses from scatteredlight and are difficult to coat. Acylinder lenses have less scattering,but require more material. Glass acylinder lenses can be anti-reflective(AR) coated to reduce loss. FIG. 8D depicts optical element 600 enabledas a compound parabolic concentrator (CPC). More explicitly, theconcentrating optics section 432 comprises a first CPC 600 with an inputaperture elongated in a transverse plane, with a plurality of opticalfunnels enabled as second CPCs or flat wedges 604 underlying the firstCPC, with input apertures elongated in the axial plane.

FIGS. 9A through 9F depict exemplary optical funnel designs. FIGS. 9Aand 9B illustrate a hollow optical funnel 604 with inner reflectivesurfaces 900. As shown in these figures, the facets or outside surfaces902 may be flat, as shown in FIG. 9B, or curved as shown in FIG. 9C. Forexample, the optical funnel 604 may have curved exterior surfaces 902shaped as a compound parabolic concentrator (CPC). Alternatively, asshown in FIGS. 9D and 9E the CPC may be a solid. In one aspect, theoptical funnels may be a dielectric material with exterior surfacefacets that are either curved or flat, that transmit R band wavelengthsof light accepted at an optical input aperture, initially by refraction,and subsequently to a corresponding receiving surface via total internalreflection (TIR), see FIG. 9F.

In one aspect, a cylinder lens with aspheric profile (also called anacylinder lens) is paired with a row of rectangular glass or plastic CPCfunnels. CPCs are non-imaging elements which optimally collect lightwithin a well-defined acceptance angle. The smaller the acceptanceangle, the greater is the potential concentration. Since the acylinderlens operates at a very fast focal ratio, the funnels are not able toprovide much additional concentration in the axial plane. However, sincethe Cassegrain trough inherently operates at a very slow focal ratio,significant additional concentration is possible in the transverseplane, especially if the funnels are solid dielectrics bonded to thecells. In the transverse plane (X-dimension), the funnels provide gapsbetween adjacent cells. These inactive regions between the cells areuseful for wiring, bypass diodes, etc.

The CPC funnels may have asymmetric acceptance angles. In theX-direction, the acceptance angle is tailored to accept all rays fromthe spectrum splitter, which subtends a relatively small angle from thevertex of the trough. In the Y-direction, the acceptance angle istailored to accept all light from the edge of the concentrating opticssection. If the concentrating optics sections operate at a fast focalratio, this angle is large and only modest additional concentration isrealized.

As shown in FIG. 5, solar radiation is concentrated in two successiveorthogonal stages. First, the primary mirror M1 (reflective trough 402)and dichroic mirror M2 (spectrum splitter 416) form a line focus ofmedium concentration along the secondary linear focus 426. Second, anarray of concentrating lens (CL) optics (i.e. concentrated opticssections 432) divide the line focus into discrete bins and performadditional concentration in the direction orthogonal to the primary linefocus. The focal length of the CLs 432 is much smaller than the focallength of the Cassegrain trough 402. Since these two stages have verydifferent focal lengths, the CL array gives a series of foci with veryhigh aspect ratios. The system 400 allows the division of theseelongated foci onto individual cells. First, a linear refractive lens,such as a cylinder lens or linear Fresnel lens, concentrates the lightorthogonal to the overall line focus. Then, a series of solid or hollowfunnels divides the light into smaller regions, each corresponding to asingle cell. These regularly-spaced cells are preferably connected inparallel to allow tolerance for irradiance variations.

FIG. 10 is a perspective drawing depicting an array of optical funnelswith varying aperture lengths. The distribution of light in thedimension of concentration performed by the trough is set by the troughprescription, optical errors, and tracking errors. The convolution ofthese errors results in a line spread function which is roughlyGaussian. When convolved with the top-hat angular spread of the sun, thedistribution is further broadened, with highest irradiance in thecenter. This would direct more light to the center cells. In order toimprove the cell-to-cell irradiance matching, the optical funnelelements may be irregularly sized such that the outer funnels havelarger input apertures in the transverse plane. This directs more lightto the edge cells, which on average are light-starved compared to thecenter cells. In order to accommodate irregular entry faces (long andshort aperture lengths), the heights of the funnels may also vary, asshown. The exit apertures for all seven funnels in this example areequal in size.

In one exemplary system, the Cassegrain trough has a focal length of7770 millimeters (mm), while the second-stage optics (dichroic spectrumsplitter) operating in the orthogonal direction has a focal length of˜50 mm. This produces a paraxial solar image with an aspect ratio of˜1:150. With aberrations in the second stage, the actual aspect ratio isless severe. Since the first stage has a very long focal ratio,additional concentration is possible in the primary concentrationdimension. This is achieved with an array of dielectric funnels that mayhave flat or curved side walls Those funnels may be compound parabolicconcentrators (CPCs). The large ends of the funnels meet edge-to-edgeand tessellate the elongated focus of the two-stage concentrator. Thesmall ends are bonded to individual PV cells. The segmentation is verybeneficial, since it provides inactive regions between cells for wiring,bypass diodes, etc.

FIGS. 11A through 11D describe the system of FIG. 5 from a differentperspective. The reflective trough is able to rotate about its primaryaxis (i.e. parallel to the trough Y-axis). The CL optics 1100 move withthe trough and rotate additionally about a local axis parallel to the CLX-axis. This rotation compensates for the residual incidence angle inthe axial plane of the trough. The CL optics tessellate the troughsecondary line focus, shown here at the base vertex of the trough. Asthe incidence angle changes, the CLs individually rotate such that theirlocal Y-axes remain nearly perpendicular to the incoming light. Thisresults in an unavoidable self-shadowing between adjacent CL elements,with the magnitude of the shadowing scaling with (1-cos θ), where θ isthe incidence angle. Only the solar rays in the R band are illustratedhere; they all reflect off the dichroic secondary (spectrum splitter)416. Solar rays in the thermal band transmitted through spectrumsplitter are absorbed by the thermal reciever. For clarity, only theoptical elements (e.g., lenses), but not the optical funnels, are shown.

The CL aperture in the X-direction (traversing the primary linear focus)is determined by the width of the aberrated solar image produced by thetrough. For full collection, the CL must be as wide as the aberratedsolar image on the winter solstice noon (the time of year which resultsin the largest incidence angle). The CL aperture in the Y-direction isdriven by mechanical and electrical considerations. If the cell groupsbelow each CL aperture are connected in parallel, then the CLY-dimension is chosen to maintain a reasonable electrical current in thecorresponding cell group.

FIGS. 12A through 12D depict the system of FIGS. 11A through 11Doperating at different angles of the sun. The outputs of the CL elements1100 are highly elongated solar images. The CL elements and theircorresponding receivers move together as a unit, rotating as a packageabout an axis parallel to the local X-axis. The CL elements may be usedto directly illuminate thin rows of PV cells 608. However, since cells608 may be arranged edge-to-edge, any gap between cells in theX-direction constitutes a loss, as shown in FIG. 12D. In the embodimentsshown here, the CL 1100 array is positioned near the trough vertex.Alternatively, the CL array may be positioned above or below the vertex.

A trough tracked about a horizontal, North-South oriented axis receiveson-axis solar radiation at multiple times throughout the year, includingthe equinox sunrise or sunset. At the summer solstice, noon, (FIGS. 12Aand 12B), the sun is almost straight over the trough with an incidentangle of |φ|·ε, where φ is the site latitude and ε=23.5°, thedeclination of Earth. For example, at Tucson Ariz., this incident angleis 8° on summer solstice noon, very close to on axis situation shown inFIGS. 12A and 12B. Therefore, the CLs 1100 are pointing almost straightup to the spectrum splitter 416 (M2). One extreme illumination conditionis noon on the Winter solstice (FIGS. 12C and 12D). In this case, theincidence angle is |φ|+ε, where φ is the site latitude and ε=23.5°, thedeclination of Earth. An off-axis (θ=45°) illumination condition isdepicted. The CL optics rotate to optimally focus the incoming lightonto the receiver surface.

As described above, in one aspect the receiving surface is tessellatedwith a row of funnel-like optics, each with its entrance aperturemeeting edge-to-edge with the adjacent funnels. The smaller exitapertures are positioned over individual PV cells. Thus, each CL unit iscomposed of an upper concentrating lens over an array of funnels. Theelements move together as a group, such that the upper lens, lowerfunnels, and PV cells maintain a fixed spatial relationship to eachother. The top optical element and bottom funnels may take on multipleembodiments, as described above.

One challenge is that the width of the solar image in the X-direction isdetermined by the Cassegrain trough. Skew dilation causes the width ofthe solar image to change throughout the year. In this case, the opticalerrors from the trough and the angular width of the sun give a solarimage which only fills a few of the center funnel/cell pairs. As theskew angle increases, with an extreme at noon on the Winter solstice,the solar image grows, filling more cells. A tradeoff must be madebetween annual collection efficiency and geometric concentration. Onepromising option is to truncate performance near the winter solsticenoon condition. The width of the CL may be shortened in the X-dimension,with fewer cells underneath. This causes some loss of light during theextreme illumination cases, but increases the average geometricconcentration for most of the year.

This disclosure describes a hybrid CSP-CPV trough solar energyconverter. The hybrid system modifies a conventional CSP system byadding a dichroic mirror, a CPV array, secondary tracking, and a thermalmanagement scheme. The CL top optical element and bottom funnels maytake on multiple configurations. Both linear Fresnel and acylinderlenses are suitable candidates for the top CL optical element.

The optical funnels may have equally-sized (length) entrance apertures.Since the irradiance profile changes in the X-direction, as a result ofthe line spread function of the trough, the cells receive unequalillumination. If the cells are wired in parallel, the effects of themismatch are mitigated. Another approach to equalizing flux betweenadjacent cells is to increase the entrance apertures of the edgefunnels. This may require using unequal height funnels.

In summary, a hybrid trough system has been presented with a reflectivetrough (M1), heat receiver (HR1), dichroic spectrum splitter (M2),concentrating lens array with PV array, tracking mechanisms in bothprimary and secondary, trough supporting frames, and a thermalmanagement scheme (HR2) to cool off CPV cells or to harvest waste heatfor field heating during night time. The CL array (the array ofconcentrating optics sections) concentrates light onto PV cells withvery high geometric concentration ratio though the orthogonal lightmanagement. The CL array is composed of top optical element and bottomfunnels, which can take on many embodiments. For example, the topoptical element can be an acylinder lens or linear Fresnel lens. Thebottom funnels may be hollow or solid, with curved sides (CPC) or flatfaceted sides.

FIG. 13 is a flowchart illustrating a solar power method using two-stagelight concentration to drive CPV conversion in conjunction with thermalcollection. Although the method is depicted as a sequence of numberedsteps for clarity, the numbering does not necessarily dictate the orderof the steps. It should be understood that some of these steps may beskipped, performed in parallel, or performed without the requirement ofmaintaining a strict order of sequence. Generally however, the methodfollows the numeric order of the depicted steps and is associated withthe system and subcomponents of the system described in FIG. 5. Themethod starts at Step 1300.

Step 1302 concentrates light rays received in a plurality of transverseplanes towards a primary linear focus in an axial plane, orthogonal tothe transverse planes. Step 1304 transmits T band wavelengths of lightto the primary linear focus. Step 1306 reflects R band wavelengths oflight towards a secondary linear focus in the axial plane, parallel tothe primary linear focus. Step 1308 translates the light received at theprimary linear focus into thermal energy. Step 1310 focuses the lightreceived at the secondary linear focus along a plurality of tertiarylinear foci, orthogonal to the axial plane. Step 1312 concentrates thefocused light in each tertiary primary focus into a plurality ofreceiving areas. Step 1314 translates the light accepted at thereceiving areas into electrical energy.

In one aspect, concentrating light rays received in Step 1302 includes areflective trough, having a primary axis and a parabolic curved surface,concentrating the light rays. In another aspect, transmitting light tothe primary linear focus (Step 1306), and reflecting light towards thesecondary linear focus (Step 1308) include using a dichroic spectrumsplitter having a hyperbolically curved surface, an axis aligned inparallel to the primary linear focus, and a position between thesecondary linear focus and the primary linear focus. The dichroicspectrum splitter transmits the T band wavelengths of light, andreflects the R band wavelengths light.

In one aspect, focusing the light received at the secondary linear focusin Step 1310 includes a plurality of corresponding optical elementfocusing the R band wavelengths of light. In another aspect,concentrating the focused light in each tertiary primary focus in Step1312 includes a plurality of optical funnels concentrating the focusedlight.

A system and method have been provided for a hybrid trough solar powersystem to use two-stage light concentration to drive CPV conversion inconjunction with a thermal collector. Examples of particularsubcomponents and components layouts have been presented to illustratethe invention. However, the invention is not limited to merely theseexamples. Other variations and embodiments of the invention will occurto those skilled in the art.

We claim:
 1. A hybrid trough solar power system using two-stage lightconcentration to drive concentrating photovoltaic (CPV) conversion inconjunction with a thermal collector, the system comprising: areflective trough having a primary axis and a parabolic curved surfacefor concentrating light rays received in a plurality of transverseplanes into a primary linear focus in an axial plane, orthogonal to thetransverse planes; a dichroic spectrum splitter having a hyperbolicallycurved surface, an axis aligned in parallel to the primary linear focus,and a position between the reflective trough and the primary linearfocus, the dichroic spectrum splitter transmitting T band wavelengths oflight, and reflecting R band wavelengths of light to a secondary linearfocus formed parallel to a vertex of the reflective trough in the axialplane; a thermal collection tube aligned along the primary linear focusfor the T band wavelengths of light; a plurality of concentrating opticssections formed in series along the secondary linear focus, eachconcentrating optics section comprising: one optical element focusingthe R band wavelengths of light reflected by the dichroic spectrumsplitter along a tertiary linear focus, orthogonal to the axial plane; aplurality of optical funnels for concentrating the R band wavelengths oflight focused by the optical element to a corresponding plurality ofreceiving areas; and, a plurality of PV devices, each having an opticalinterface formed at one of the plurality of corresponding receivingareas.
 2. The system of claim 1 wherein each optical element has anoptical input aperture elongated orthogonal to the axial plane; and,wherein the plurality of optical funnels in each concentrating opticssection each have an optical input aperture underlying the opticalelement and elongated orthogonal to the axial plane.
 3. The system ofclaim 2 wherein each optical element has an optical input aperture firstaxial plane-width in an aperture plane, where the aperture plane isorthogonal to the axial plane, and each optical element focusesreflected R band wavelengths of light along the tertiary linear focus toa second axial plane-width, smaller than the first axial plane width. 4.The system of claim 3 wherein each concentrating optics section isrotatable about a corresponding local axis, each local axis formedorthogonal to the axial plane.
 5. The system of claim 2 wherein eachoptical element is selected from a group consisting of Fresnel lens,cylindrical lens, and acylindrical lens.
 6. The system of claim 1wherein each optical funnel is hollow with inner reflective surfaces,with facets selected from a group consisting of curved and flat.
 7. Thesystem of claim 1 wherein each optical funnel has curved exteriorsurfaces shaped as a compound parabolic concentrator (CPC).
 8. Thesystem of claim 1 wherein each optical funnel is a dielectric materialwith exterior surface facets selected from a group consisting of flatand curved, transmitting R band wavelengths of light accepted at anoptical input aperture, initially by refraction, and subsequently to acorresponding receiving surface via total internal reflection (TIR). 9.The system of claim 1 where the PV devices are selected from the groupconsisting of single-junction and multi-junction cells, each junctionhaving an energy bandgap converting R band wavelengths of light toelectrical current.
 10. The system of claim 9 wherein the dichroicspectrum splitter reflects light in the R band of wavelengths between500 and 810 nanometers (nm); and, wherein the PV devices are selectedfrom a group consisting of double junction tandem cells with energybandgaps of 1.88 electron volts (eV) and 1.43 eV, and triple junctiontandem cells with an energy bandgaps of 2.05 eV, 1.77 eV, and 1.43 eV.11. The system of claim 9 wherein the dichroic spectrum splitterreflects light in the R band of wavelengths between 650 and 850 nm; and,wherein the PV devices are single junction cells with an energy bandgapof 1.43 eV.
 12. The system of claim 9 wherein the dichroic spectrumsplitter reflects light in the R band of wavelengths between 700 and1000 nm; and, wherein the PV devices are single junction cells with anenergy bandgap of 1.1 eV.
 13. The system of claim 1 wherein thereflective trough is rotatable about the primary axis.
 14. The system ofclaim 1 further comprising: thermal cooling blocks attached to the PVdevices.
 15. The system of claim 1 wherein dichroic spectrum splittertransmits T band wavelengths of light both greater than near infrared(NIR) and less than NIR, and reflects R band wavelengths in the NIRwavelengths of light.
 16. A solar power system using two-stage lightconcentration to drive concentrating photovoltaic (CPV) conversion, thesystem comprising: a plurality of concentrating optics sections formedin series along a secondary linear focus, each concentrating opticssection comprising: an imaging optical element focusing R bandwavelengths of light along a tertiary linear focus orthogonal to anaxial plane; a plurality of optical funnels aligned serially in a rowalong the tertiary linear focus of the imaging optical element, theplurality of optical funnels concentrating the R band wavelengths oflight focused by the imaging optical element to a correspondingplurality of receiving areas; for each optical funnel, a PV devicehaving an optical interface formed at the corresponding receiving areaof the plurality of corresponding receiving areas; wherein eachconcentrating optics section is independently rotatable about acorresponding local axis, and each local axis is orthogonal to the axialplane; and, wherein the orthogonality of each imaging optical element tothe R band wavelengths of light is responsive to the rotation of acorresponding concentrating optics section about its local axis.
 17. Thesystem of claim 16 wherein each imaging optical element has an opticalinput aperture elongated orthogonal to the axial plane; and, wherein theplurality of optical funnels in each concentrating optics section eachhave an optical input aperture underlying the corresponding imagingoptical element and elongated orthogonal to the axial plane.
 18. Thesystem of claim 17 wherein each imaging optical element has an opticalinput aperture first axial plane-width in an aperture plane, where theaperture plane is orthogonal to the axial plane, and each imagingoptical element focuses light along the tertiary linear focus to asecond axial plane-width, smaller than the first axial plane width. 19.The system of claim 17 wherein each imaging optical element is selectedfrom a group consisting of Fresnel lens, cylindrical lens, andacylindrical lens.
 20. The system of claim 17 wherein each opticalfunnel is hollow with inner reflective surfaces, with facets selectedfrom a group consisting of curved and flat.
 21. The system of claim 17wherein each optical funnel has curved exterior surfaces shaped as acompound parabolic concentrator (CPC).
 22. The system of claim 17wherein each optical funnel is a dielectric material with exteriorsurface facets selected from a group consisting of flat and curved,transmitting R band wavelengths of light accepted at an optical inputaperture, initially by refraction, and subsequently to a correspondingreceiving surface via total internal reflection (TIR).
 23. The system ofclaim 17 where the PV devices are selected from the group consisting ofsingle-junction and multi-junction cells, each junction having an energybandgap converting R band wavelengths of light to electrical current.24. The system of claim 23 wherein the PV devices are selected from agroup consisting of double junction tandem cells with energy bandgaps of1.88 electron volts (eV) and 1.43 eV, and triple junction tandem cellswith an energy bandgaps of 2.05 eV, 1.77 eV, and 1.43 eV.
 25. The systemof claim 23 wherein the PV devices are single junction cells with anenergy bandgap selected from the group consisting of 1.43 eV and 1.1 eV.26. The system of claim 17 further comprising: thermal cooling blocksattached to the PV devices.
 27. The system of claim 21 furthercomprising: a reflective trough having a primary axis and a paraboliccurved surface for concentrating light rays received in a plurality oftransverse planes into a primary linear focus in the axial plane,orthogonal to the transverse planes; a dichroic spectrum splitter havinga hyperbolically curved surface, an axis aligned in parallel to theprimary linear focus, and a position between the reflective trough andthe primary linear focus, the dichroic spectrum splitter transmitting Tband wavelengths of light, and reflecting the R band wavelengths oflight to a secondary linear focus formed parallel to a vertex of thereflective trough in the axial plane; a thermal collection tube alignedalong the primary linear focus for the T band wavelengths of light; and,wherein the imaging optical elements focus the R band wavelengths oflight reflected by the dichroic spectrum splitter.